The micro initial form of the pollution flashover on the surface of insulating equipment determines its macroscopic development process. A short-gap AC pollution flashover experiment was carried out in this study. By analyzing the optical images, infrared thermal characteristics and electrical signals in the discharge process, the on-line diagnosis of the discharge development modes of polluted insulators under different humidity is realized. Among them, with the increase of voltage, the discharge presents a new mode: partial arc-surface discharge-arc, which is found under appropriate humidity. And the arc will not recover in this mode even if the voltage continues to rise within a certain range, which is considered to be the effect of thermal effect on a long time scale. This discovery provides a new perspective and idea for the research on the flashover mechanism of equipment insulators and on-line anti-pollution flashover treatment.